The fact is that the correspondence between colour and frequency is rough and approximate. To some "colours" (and what does this mean?) there corresponds no wavelength, or no single wavelength, of monochromatic light. Examples are the browns, the appropriately named "non-spectral" purples, and white. (Black is also an example!) The colours form a circle (roughly) or a three-dimensional solid of which the circle is a cross-section at middle brightness in fixed illumination. This is colour space; and the frequency scale does not really model its overal complexity, except around the one corner at the edge: the spectrum. This is related to the fact that there are three types of cone which respond to coloured light, not four. There is no photoreceptor which peaks in the yellow, so when we see yellow it is the "red" and "green" cones that are being stimulated. Scientists tend to draw the conclusion that "colour is a sensation", as Maxwell put it. But this can't be right, as I see it, for the most part for the reasons given in Thomas Reid's philosophy of sensation and perception. The relation between colour and frequncy or wavelength has been an issue since the wave theory of light was introduced by Thomas Young, but the problem was there for Newton even though he held a corpuscular theory of light; the big particles stirred up red or "red" sensations (_n.b._ red sensations), and the little ones blue ones, but the "Rays" (paths?) made of these corpuscles were "strictly" not coloured.